1. Field of the Invention
The present invention relates generally to web printing presses and more particularly to a method and apparatus for dynamically controlling a driver of a nip roller in a web printing press.
2. Background Information
Web printing presses print a continuous web of material, such as paper. Tension in the web must be maintained within a desired range in order to achieve smooth operation of the printing press. Paper differences, environmental conditions in the pressroom, as well as press evolutions such as web splicing and blanket washing, may cause web tension variations. At the same time, the velocity of the web, and hence the rotational speed of non-slip nip rollers moving the web, must be held relatively constant to achieve good print product quality. Some prior printing presses have controlled the speed of the non-slip nip rollers. These presses require the intervention of a skilled operator to achieve acceptable web tensions. Some prior printing presses have controlled the web tension. These presses may have compromised print product quality.
European Patent Document No. EP 0 933 201 A1 describes a method for controlling the nip roller driver in a web printing press based on a load torque adjusted according to load characteristic curve. hi this way, the driver purportedly produces a desired rotational speed of the nip roller and at the same time a desired web tension.
An object of the present invention is to provide a web printing press control scheme which changes according to the phase of printing press operation.
The present invention provides a method for dynamically controlling a driver of a nip roller in a web printing press, the method comprising: controlling a speed of the driver in a tension-control mode during a first phase of printing press operation, the controlling of the speed of the driver in the tension-control mode being based on a tension in the web upstream of the nip roller so as to maintain the tension at a first desired tension value; and controlling the speed of the driver in a velocity-control mode during a second phase of printing press operation, the controlling of the speed of the driver in the velocity-control mode being based on a driver velocity ratio.
According to the present invention, the control mode of the nip roller driver is thus adapted to the particular operating phase of the printing press.
The tension maintained according to the present invention may be a measured or estimated tension in the upstream web. The desired tension value may be derived from an operator input or from a previously determined set of tension set points which are a function of a type of paper of the web and/or an operating condition of the web printing press.
The first phase of printing press operation may include acceleration of the nip roller driver to a steady-state speed during a start-up of the printing press. Tension-control of the driver speed is desirable during start-up and acceleration to make-ready speed since printing has not yet commenced and print quality is therefore not an issue.
The second phase of printing press operation may include a printing operation of the printing press. Once printing has started, velocity control of the nip roller driver is implemented to maintain print product quality.
The driver velocity ratio used in the velocity-control mode is a ratio of the speed of the driver needed to produce a given desired web tension value to the value of the command, or nominal, velocity of the web printing press.
The driver velocity ratio may be determined during a transition phase of printing press operation when the driver is at the steady-state speed.
The method according to the present invention may further comprise: controlling the torque of the driver during a webbing-up phase of printing press operation at a torque value a given amount above a breakaway torque and below a web breaking torque; and controlling the speed of the driver during the webbing-up phase below a maximum velocity limit so as to prevent a driver overspeed condition when the tension in the web is low or zero.
During the webbing-up phase, the driver torque may thus be set to provide measurable tension in the upstream web span, but low enough to avoid web breaking. Since there may be no web yet present at the beginning of the webbing-up phase, an upper driver velocity limit is imposed to prevent overspeed of the driver when there is no web tension to match the driver torque. The press may remain in the webbing-up mode until measurable web tension had been developed.
Furthermore, according to the present invention, control of the driver speed may be switched from the velocity-control mode to the tension-control mode. This may be desirable when the web tension moves, or is likely to move, outside of an acceptable web tension limit band, or periodically. Times when the web tension is likely to move outside of an acceptable tension limit band include press blanket wash cycles and web splicing cycles. Also, the web tension may move out of an acceptable tension limit band due to temperature, humidity and paper changes. Additionally, control of the driver speed may be switched from the velocity-control mode to the tension-control mode in response to an operator input, or simply periodically.
Once the driver control has been switched back to velocity-control mode, a revised velocity ratio may be determined during a time when the driver is at a steady-state speed. Then driver control may be switched back to velocity-control mode using the new velocity ratio.
Furthermore, to reduce the likelihood of a web break, driver control may be switched to tension-control mode in the event of an emergency stop of the printing press.
The present invention also provides an apparatus for dynamically controlling a driver of a nip roller in a web printing press, the apparatus comprising a tension control device for modifying a speed of the driver based on a tension in the web upstream of the nip roller in a tension-control mode during a first phase of printing press operation so as to maintain the tension at a first desired tension value; and a speed control device for controlling the speed of the driver in a velocity-control mode during a second phase of printing press operation, the controlling of the speed of the driver in the velocity-control mode being based on a driver velocity ratio.